Electroluminescent displays are used as light emitting type electronic display device (ELD). As the constituting element of the ELD, an inorganic electroluminescent element (inorganic EL element) and an organic electroluminescent device (organic EL device) are cited. The inorganic EL device has been used as a planar light source though high alternative voltage is required for driving such the light emitting device.
The organic EL device is an element having a light emission layer placed between a cathode and an anode, in which electrons and holes are injected into the light emission layer and excitons are generated by recombination of them, and light (fluorescence or phosphorescence) is emitted on the occasion of quenching of the excitons. Such the device is noted because which can emit light by application of a voltage of several to several tens volts, and has wide viewing angle and high visibility since it is a self light emission type, and is completely solid state thin device suitable for space saving and portable appliance.
It is important character of the organic EL device that the element is a planar light source different from ordinary practically used principal light sources such as light emission diodes and cold cathode ray tubes. As the use for effectively utilizing such the character, illuminating light source and backlight for various displays are cited. Particularly, such the element is suitably used for the backlight of liquid crystal full color display which is strongly demanded in recent years.
An improvement of light emission efficiency is required to utilize the organic EL element practically in illuminating light source or backlight for display. Function-separation has become general so that an organic functional layer composing the organic EL element is composed of plural layers each having individual functional materials, or, is composed of mixture of the materials to improve the light emission efficiency.
Manufacturing method of an organic EL element include an evaporation method, a wet process such as a spin coat method, a cast method, a die coat method, an inkjet method, a spray method and a printing method, (hereafter referred to a coating method), and the manufacturing method via wet process is marked recently since continuous production is easily conducted without using a process in a vacuum.
However, in case of the organic EL element manufactured via wet process there is restriction particularly superposing same compounds in layers such as scraping out or washing out of the lower layer in the coating method in comparison with so called evaporation method. Therefore, function-separation is not attained in case of manufacturing the organic electronic device such as an organic EL element in comparison with the evaporation method.
An idea is proposed to provide an electron transportable hole block layer between an electron transport layer and a light emission layer, effects to trap a hole within light emission layer appears, and as its result recombination probability between hole and an electron is improved to enhance a light emission efficiency as a means for improving Performance of an organic EL element in an organic EL element (for example, Patent Document 1).
An idea is proposed to form a concentration gradient in a thickness direction by mixing electron injection martial with electron transportable material, and carrier injection performance between layers is enhanced to reduce power consumption of the device as well as to elongate life time (for example, Patent Document 2).
However, these can be easily produced via an evaporation method, it is impossible to form plural layers containing a common compound using the same solvent via a coating method since the lower layer is dissolved. Further it is difficult to control so as to give gradient of mixture ratio of plural materials in a thickness direction.
To dissolve the problems, an idea is proposed in which mixture area of certain functional layer is provided in at least two layers among organic layers, and at least one composing component has continuous concentration gradient in concentration distribution whereby element having a high light emission efficiency, low driving voltage and further long emission life time via a coating method (for example, Patent Documents 3 and 4).
However, there is a problem not to applied means to form plural layers using same compound since these are utilized the difference of solubility of material between two adjacent layers in this instance. Further it is possible to relaxing the barrier at the interface by continuously mixing the component between two layers, on the other side it reduces block function of carrier or exciton, and therefore it is difficult to attain performance similar to evaporation method.